CALIPSOS is a steady-state three-dimensional flow distribution code which predicts the fluid dynamics and heat transfer interactions of the secondary two-phase flow in a steam generator. The mathematical formulation is sufficiently general to accommodate two fluid models described by separate gas and liquid momentum equations. However, if the user selects the homogeneous flow option, the code automatically equates the gas and liquid phase velocities (thereby reducing the number of momentum equations solved to three) and utilizes a homogeneous density mixture. This report presents the basic features of the CALIPSOS code and includes assumptions, equations solved, the finite-difference grid, and highlights of the solution procedure.